---
id: CVE-2026-100724
title: >-
  http4k (Maven package org.http4k:http4k-core) before 6.49.0.0, 5.42.0.0 and
  4.51.0.0 uses substring (Contains) matching on the Host header by default in
  reverseProxy() and reverseProxyRouting() when dispatching to configured
  virtual host…
summary: >-
  http4k (Maven package org.http4k:http4k-core) before 6.49.0.0, 5.42.0.0 and
  4.51.0.0 uses substring (Contains) matching on the Host header by default in
  reverseProxy() and reverseProxyRouting() when dispatching to configured
  virtual host…
severity: medium
cvss: 5.4
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N'
cwe:
  - CWE-444
vendor: http4k
product: http4k
affected:
  - http4k < 6.49.0.0
  - http4k < 5.42.0.0
  - http4k < 4.51.0.0
published: '2026-09-27'
updated: '2026-09-27'
sourceUpdated: '2026-09-27T02:17:20.603'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-100724'
references:
  - url: 'https://github.com/http4k/http4k/commit/0121b05537'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/http4k/http4k/commit/54c6385615'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/http4k/http4k/security/advisories/GHSA-jrpc-7vxp-69p6'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/http4k-before-6.49.0.0-host-header-routing-bypass-via-reverseproxy
    label: disclosure@vulncheck.com
tags:
  - nvd
  - cve.org
ingestedAt: '2026-09-27T02:41:45.566Z'
---

## Overview

http4k (Maven package org.http4k:http4k-core) before 6.49.0.0, 5.42.0.0 and 4.51.0.0 uses substring (Contains) matching on the Host header by default in reverseProxy() and reverseProxyRouting() when dispatching to configured virtual hosts. If these functions are deployed as a public-facing inbound HTTP handler with two or more configured virtual hosts, a remote attacker can supply a Host header that merely contains a configured vhost name (for example Host: admin.evil.com for a vhost configured as "admin") and be routed to that vhost, bypassing routing-based authorization. The intended outbound-dispatch and test-time uses, where the Host value is set by the calling application, are not affected.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.